Synthetically produced polymers such as polydienes are used in the art of manufacturing tires. Synthetic polymers that undergo strain-induced crystallization provide advantageous properties such as tensile strength and abrasion resistance. Thus, cis-1,4-polydienes with high cis-1,4-linkage content, which exhibit the increased ability to undergo strain-induced crystallization, have been advantageously employed. Also, certain functionalized polymers have been used in the manufacture of tires to prepare vulcanizates that demonstrate reduced hysteresis, i.e., less loss of mechanical energy to heat. It is believed that the functional group of the functionalized polymers reduces the number of free polymer chain ends via interaction with filler particles and may also reduce filler agglomeration. Thus, cis-1,4-polydienes have advantageously been functionalized to provide vulcanizates that undergo strain-induced crystallization and demonstrate reduced hysteresis. The ability to functionalize the polymer, particularly at its chain end, depends on the reactivity of the polymer. Typically, only a fraction of the polymer molecules in any given sample can be reacted with functionalizing agents. It is therefore desirable to develop a method for producing cis-1,4-polydienes having higher cis-1,4-linkage content and a greater percentage of reactive chain ends for functionalization.
Polydienes may be produced by solution polymerization, wherein conjugated diene monomer is polymerized in an inert solvent or diluent. The solvent serves to solubilize the reactants and products, to act as a carrier for the reactants and product, to aid in the transfer of the heat of polymerization, and to help in moderating the polymerization rate. The solvent also allows easier stirring and transferring of the polymerization mixture (also called cement), since the viscosity of the cement is decreased by the presence of the solvent. Nevertheless, the presence of solvent presents a number of difficulties. The solvent must be separated from the polymer and then recycled for reuse or otherwise disposed of as waste. The cost of recovering and recycling the solvent adds greatly to the cost of the polymer being produced, and there is always the risk that the recycled solvent after purification may still retain some impurities that will poison the polymerization catalyst. In addition, some solvents such as aromatic hydrocarbons can raise environmental concerns. Further, the purity of the polymer product may be affected if there are difficulties in removing the solvent.
Polydienes may also be produced by bulk polymerization (also called mass polymerization), wherein conjugated diene monomer is polymerized in the absence or substantial absence of any solvent, and, in effect, the monomer itself acts as a diluent. Since bulk polymerization is essentially solventless, there is less contamination risk, and the product separation is simplified. Bulk polymerization offers a number of economic advantages including lower capital cost for new plant capacity, lower energy cost to operate, and fewer people to operate. The solventless feature also provides environmental advantages, with emissions and waste water pollution being reduced.
Despite its many advantages, bulk polymerization requires very careful temperature control, and there is also the need for strong and elaborate stirring equipment since the viscosity of the polymerization mixture can become very high. In the absence of added diluent, the high cement viscosity and exotherm effects can make temperature control very difficult. Consequently, local hot spots may occur, resulting in degradation, gelation, and/or discoloration of the polymer product. In the extreme case, uncontrolled acceleration of the polymerization rate can lead to disastrous “runaway” reactions. To facilitate the temperature control during bulk polymerization, it is desirable that a catalyst gives a reaction rate that is sufficiently fast for economical reasons but is slow enough to allow for the removal of the heat from the polymerization exotherm in order to ensure the process safety.
Lanthanide-based catalyst systems that comprise a lanthanide-containing compound, an alkylating agent, and a halogen source are known to be useful for producing conjugated diene polymers having high cis-1,4-linkage contents. Nevertheless, when applied to bulk polymerization of conjugated dienes, lanthanide-based catalyst systems, especially those comprising an aluminoxane compound as a catalyst component, often give excessively fast polymerization rates, which makes it very difficult to control the temperature and compromises the process safety. Therefore, it is desirable to develop a method of moderating the bulk polymerization of conjugated dienes catalyzed by lanthanide-based catalysts.